The present invention relates to a device for producing a mixture from chemically reactive plastic components and permeated with reinforcement fibers.
A device of this type is described, for example, in German Pat. No. DE 196 18 393 in which the reinforcement fibers are made of glass fibers or similarly brittle fibers. The fibers are hereby engaged by a pair of feed rollers and drawn from a continuous fiber strand. One of the feed rollers is equipped with one or more blades so that the fibers are subdivided in pieces of given length during passage through the feed rollers. The fiber pieces cut and accelerated with the feed rollers are transported at high velocity through a central channel in the cleaning piston of a polyurethane mixing head into the mixture of chemically reactive plastic components generated in the mixing head. The cutting of the continuous fibers in pieces of given length at extremely high velocity can be implemented by this device only with relatively brittle fibers, such as, e.g. glass fibers, in which the blade impacting transversely upon the fiber strand causes an immediate breaking of the fibers.
As their disposal is problematic, the application of glass fibers becomes increasingly controversial in many technical fields, e.g. interior area of motor vehicles, so that there exists a pressing need to replace the glass fibers by natural fibers, e.g. hemp fibers. Due to the soft-elastic and tough consistency of natural fibers, the cutting mechanism known from German Pat. No. DE 196 18 393 is unsuitable for producing pieces of natural fibers, such as, e.g., hemp fibers, through the required high-velocity process.
It would therefore be desirable and advantageous to provide an improved device for producing a mixture from chemically reactive plastic components and permeated with reinforcement fibers, obviating prior art shortcomings and suitable for integration in a mixing head of a polyurethane apparatus, for drawing fibers with soft-elastic and tough characteristics (natural fibers) at high velocity from a continuous strand and introduction into the mixture stream of chemically reactive plastic components in the mixing head, after the cutting into fiber pieces of random length.
The present invention provides for a device for producing a mixture from chemically reactive plastic components and permeated with reinforcement fibers, with the device including a mixing head having a mixing chamber receiving chemically reactive plastic components for producing a plastic mixture; an outlet tube disposed downstream of the mixing chamber for receiving the plastic mixture, a cleaning piston reversibly displaceable in the outlet tube and having a central channel, a conveying unit for feeding a strand of natural fibers to the cutting arrangement, and a cutting arrangement including an outer tube which defines an axis and terminates in an outlet port, an inner guide tube receiving the strand of natural fibers and surrounded by the outer tube at formation of a gas supply channel of ring-shaped cross section for conduction of compressed gas, wherein the inner tube and the outer tube are so configured as to form a ring nozzle, with the outer tube extending beyond a lower end of the inner tube for guiding the strand of natural fiber downstream of the ring nozzle, a cutting mechanism having at least one knife element extending transversely to the axis of the outer tube and in shearing contact with an end surface of the outer tube pass when passing by the outlet port, and an exit tube arranged downstream of the knife element in communication with the central channel of the cleaning piston.
In accordance with the present invention, very soft-elastic natural fibers can now be held in optimum manner in straight extension through application of a vacuum in the entry zone and through feed to the cutting edges in a compressed gas stream, so that the cut nature fiber pieces can be fed, like cut glass fiber components, together with the compressed gas stream in targeted manner to the mixture stream of chemically reactive plastic components.
According to another feature of the present invention, the mixing head includes a housing and a dip tube which is securely fixed to the housing, said dip tube traversing the central channel of the cleaning piston and communicating with the exit tube. Suitably, the dip tube is securely fixed to the exit tube.
According to another feature of the present invention, the knife element includes a knife disk which is driven about an axis in parallel relationship to the axis of the outer tube and has at least one kidney-shaped through-opening having at least one rounded area forming a cutting edge.
The cutting arrangement may include a cutting drum and a retainer ring for replaceably securing the knife element to the cutting drum, whereby the cutting drum has a central opening expanding conically upwards to the through-opening of the knife disk and tapering downwards to an opening terminating in the exit tube. Suitably, the cutting drum is supported in the housing and driven by a drive mechanism supported by the housing and including a toothed belt in driving relationship with the cutting drum, wherein the housing has a cutout for passage of the tooth belt.